According to a news release regional haze in Wyoming is caused mostly by sulfur dioxide (SO2) and particulate matter (PM). NOx-based constituents make up a very small part of the haze. The EPA approved Wyoming’s plans for SO2 and particulate matter.

This argument is about visibility conditions and Wyoming’s plan ensures progress. Environmental groups are pushing for the most strict NOx controls, at extraordinary cost, and that make little perceptible difference in visibility conditions. Their arguments are disingenuous,” Governor Mead said.

The EPA approved Wyoming’s plans for SO2 and PM. The 10th Circuit Court of Appeals has upheld Wyoming’s SO2 plan. Wyoming’s PM plan was not challenged.